Carbon steel wire and method for manufacturing same

ABSTRACT

Provided is a carbon steel wire excellent in shear resistance as compared with a conventional one and a method of manufacturing such a carbon steel wire. Provided is a carbon steel wire  1  having a wire diameter of from 0.1 to 0.6 mm, and when the radius of a circular cross-section orthogonal to the longitudinal direction is r and a region from the outer periphery of the circular cross-section toward the center to 0.4r is a surface layer portion  2 , the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion  2  is 60% or less.

TECHNICAL FIELD

The present invention relates to a carbon steel wire and a method of manufacturing the same, and particularly to a carbon steel wire excellent in shear resistance as compared with a conventional one and a method of manufacturing such a carbon steel wire.

BACKGROUND ART

Conventionally, a steel wire used for reinforcing rubber articles such as tires and high pressure hoses is manufactured by drawing a carbon steel wire rod containing about 0.70 to 0.90% by mass of carbon to a predetermined intermediate wire diameter and then applying a thermal treatment and a brass plating treatment to an intermediate wire rod, and further, drawing this intermediate wire rod to a final wire diameter. When used for reinforcing rubber articles, such a carbon steel wire is embedded in unvulcanized rubber as a single cord or a steel cord obtained by twisting, and vulcanization of rubber and bonding of the carbon steel wire and the rubber are carried out by heating.

In recent years, with a growing demand for energy saving and resource saving, development of a higher strength carbon steel wire is desired. In order to manufacture a high strength carbon steel wire by a manufacturing method as described above, the wire drawing amount to be applied to the carbon steel wire rod is needed to be increased. However, when the wire drawing amount is increased, the ductility of the carbon steel wire is decreased, and problems such as breakage during manufacturing or deterioration of durability at the time of use tend to occur. In particular, decrease in ductility of a surface layer portion may be a dominant factor with respect to a possible wire drawing amount, or an achievable strength. This is because a distortion due to wire drawing tends to concentrate on the surface layer portion of the carbon steel wire rather than the inside thereof, and the surface layer portion cannot bear the strong processing earlier than an inside portion. Age hardening or poor lubrication due to heat generation by friction with a die is further added, which promotes deterioration of ductility of the surface layer portion. In order to solve such a problem of deterioration of ductility, improvements have been made on wire drawing technique (for example, Patent Document 1).

RELATED ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Unexamined Patent Application     Publication No. 10-325089

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Today it is known that the strength of the steel cord obtained by twisting carbon steel wires is not the sum of the strength of the carbon steel wires which are constituent elements thereof. This is because when a tension is applied, a shearing force caused by twisting of a part of carbon steel wires in a steel cord becomes strong, and the carbon steel wire in that part causes a preceding fracture. As a result, the strength of the steel cord is considerably lower than the sum of the strengths of the carbon steel wires. Therefore, in order to further improve the strength of a steel cord, realization of a technique to solve a problem of a preceding fracture of a carbon steel wire is demanded.

Accordingly, an object of the present invention is to provide a carbon steel wire excellent in shear resistance as compared with a conventional one, which is hardly subject to preceding fracture even when twisted into a steel cord, and a method of manufacturing such a carbon steel wire.

Means for Solving the Problems

In order to solve the above-described problems, the inventor intensively studied to obtain the following findings. Specifically, a shear-resistant carbon steel wire having a low strength of about 2,750 MPa causes a large plastic deformation which causes necking against a shear, thereby alleviating a shear stress. On the other hand, the present inventor found that, since a carbon steel wire having a high strength of, for example, 3,500 MPa can hardly be plastically deformed, a shearing load concentrates on one point, thereby leading to fracture at this point. Based on such findings, the present inventor further intensively studied to find that the plastic deformation capacity of a carbon steel wire can be retained by setting a distribution of a crystal texture in a circular cross-section orthogonal to the longitudinal direction of the carbon steel wire to a predetermined distribution, whereby the shear resistance of the carbon steel wire can be improved, thereby completing the present invention.

In other words, a carbon steel wire of the present invention is a carbon steel wire having a wire diameter of from 0.1 to 0.6 mm, characterized in that,

when the radius of a circular cross-section orthogonal to the longitudinal direction is r and a region from the outer periphery of the circular cross-section toward the center to 0.4r is a surface layer portion, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion is 60% or less. Here, the crystal texture in the [110] orientation refers to a crystal texture which orients in a direction within 10° from the [110] direction when a longitudinal cross-section of a carbon steel wire is analyzed by Electron Backscatter Diffraction (EBSD).

In the carbon steel wire of the present invention, preferably, when a region inside the surface layer portion in the circular cross-section is a central portion, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the central portion is greater than 60%. The carbon steel wire of the present invention can be suitably used for reinforcing rubber articles.

A method of manufacturing a carbon steel wire of the present invention is a method of manufacturing a carbon steel wire comprising a step of subjecting a carbon steel wire rod to wet wire drawing processing by using a plurality of dies, characterized in that

when the tensile strength of an eventually obtained carbon steel wire is T (MPa) and the number of dies used in the wet processing is n, a relationship represented by the following formula (1):

T/n≦155 (MPa)  (1)

is satisfied, and when the radius of the carbon steel wire rod before the wet wire drawing processing is d0 and the wire diameter of the carbon steel wire after the wet wire drawing is d1, the maximum value of a die drag at a die with a wire drawing distortion a represented by the following formula (2):

ε=2×ln(d0/d1)  (2)

of 1.5 or less is 750 MPa or less. Here, the term “die drag” refers to a value calculated by (a tension applied to a wire passing through a die exit)−(a back tension applied to a wire before entering a die entrance).

Effects of the Invention

According to the present invention, a carbon steel wire excellent in shear resistance as compared with a conventional one and a method of manufacturing such a carbon steel wire can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a circular cross-section orthogonal to the longitudinal direction of a carbon steel wire of the present invention.

FIG. 2 is a longitudinal cross-sectional view of a carbon steel wire indicating measurement points of EBSD.

FIG. 3 is a graph illustrating relationships between distances from the surfaces of carbon steel wires of Example 1 and Comparative Example 2 and the occupancy ratios of a crystal texture in the [110] orientation with respect to the longitudinal direction of the carbon steel wires.

FIG. 4 is a schematic explanatory view of a method of measuring the shear breaking strength of a steel wire.

MODE FOR CARRYING OUT THE INVENTION

A carbon steel wire of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a cross-sectional view of a circular cross-section orthogonal to the longitudinal direction of a carbon steel wire of the present invention. A carbon steel wire of the present invention is a carbon steel wire 1 having a wire diameter of from 0.1 to 0.6 mm, and when the radius of a circular cross-section orthogonal to the longitudinal direction is r and a region from the outer periphery of the circular cross-section toward the center to 0.4r is a surface layer portion 2, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion 2 is 60% or less. In other words, by making the occupancy ratio of a crystal texture in the [110] orientation in the surface layer portion 2 smaller than conventional one, the ductility of the surface layer portion 2 is secured, the plastic deformation ability is maintained, and the shear resistance is improved. In a carbon steel wire of the present invention, in order to better obtain an effect of the present invention, the occupancy ratio of the crystal texture in the [110] orientation with respect to the longitudinal direction in a region of 0.4r from the outer periphery of a circular cross-section orthogonal to the longitudinal direction is preferably 60% or less.

In a carbon steel wire of the present invention, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in a central portion preferably exceeds 60%. Here, the central portion 3 is a region inside the surface layer portion 2 in a circular cross-section. In a carbon steel wire of the present invention, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion 2 is 60% or less, which is smaller than in the past, and therefore, although the shear resistance is improved, the tensile strength is reduced accordingly. Therefore, in order to secure the tensile strength of the carbon steel wire, the strength of the central portion 3 of the carbon steel wire is increased. In a carbon steel wire of the present invention, preferably, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the central portion 3 is 80% or more.

A carbon steel wire of the present invention is not particularly limited as long as the wire diameter is from 0.1 to 0.6 mm and the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion 2 is 60% or less. For example, for a material, a high carbon steel wire having a carbon content of 0.70 mass % or more is suitable.

A carbon steel wire of the present invention attains both tensile strength and shear resistance at a higher level than conventional one and can be suitably used for reinforcing rubber articles such as tires, belts, air springs and hoses. For example, when a carbon steel wire of the present invention is used as a reinforcing material for a tire, it can be used as a reinforcing material such as a carcass ply, a belt layer, a belt reinforcing layer, a reinforcing layer around a belt such as a flipper.

Next, a method of manufacturing a carbon steel wire of the present invention will be described in detail.

A method of manufacturing a carbon steel wire of the present invention is a method of manufacturing a carbon steel wire comprising a step of performing a wet wire drawing processing on a carbon steel wire rod by using a plurality of dies. In a method of manufacturing a carbon steel wire of the present invention, when the tensile strength of an eventually obtained carbon steel wire is T(MPa) and the number of dies used for wet wire drawing processing is n, the following formula (1):

T/n≦155 (MPa)  (1)

is satisfied.

As described above, in the carbon steel wire of the present invention, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion 2 is set to 60% or less. Such a carbon steel wire can be manufactured by retarding the [110] orientation with respect to the longitudinal direction of a crystal texture of the surface layer portion 2 of the carbon steel wire. For this purpose, it is only needed to mainly process the surface of the carbon steel wire rod in wet wire drawing processing. Therefore, it is better to use many dies so that the surface of a carbon steel wire rod frequently comes into contact with a die which is a processing jig. Therefore, in a method of manufacturing a carbon steel wire of the present invention, a value obtained by dividing the tensile strength (MPa) of an eventually obtained carbon steel wire by the number of dies used for wire drawing processing is set to 155 (MPa) or less as a reference thereof. The value is suitably 150 (MPa) or less.

Further, in a method of manufacturing a carbon steel wire of the present invention, when the radius of the carbon steel wire rod before the wet wire drawing processing is d0 and the wire diameter of the carbon steel wire after the wet wire drawing is d1, the maximum value of a die drag at a die with a wire drawing distortion ε represented by the following formula (2):

ε=2×ln(d0/d1)  (2)

of 1.5 or less is 750 MPa or less. Orientation of a crystal occurs at an initial stage of drawing processing. Therefore, it is effective to reduce the processing amount in the first half of wire drawing processing. Accordingly, in a method of manufacturing a carbon steel wire of the present invention, the maximum value of a die drag in a die having a wire drawing distortion of 1.5 or less is set to 750 MPa or less. The value is suitably 700 MPa or less.

Supplementally, when the wire drawing distortion is 2.5 or more, the orientation of the metallographic structure to the [110] orientation with respect to the longitudinal direction is almost completed. Although the crystal texture in the [110] orientation is nearly perfectly aligned in the wire drawing direction and the tensile strength increases as the fiber spacing decreases with the processing amount, ductility values such as elongation and drawing are unambiguously reduced. For this reason, a steel wire having a tensile strength of 3,000 MPa or more with a drawing distortion of 2.5 or more weakens against a shearing force.

The method of manufacturing a carbon steel wire of the present invention is not particularly limited as long as it satisfies the above-described manufacturing conditions in a wet wire drawing process. For example, as a carbon steel wire rod subjected to wire drawing, one containing 0.70% by mass of carbon can be suitably used. A processing method, processing conditions, or the like of the above-described wet wire drawing processing can be appropriately designed according to a usual method as desired. Further, there are no particular restrictions on various processes performed prior to the above-described wet wire drawing processing process, and after carrying out processes such as dry drawing, patenting heat treatment and plating treatment, a method of manufacturing a carbon steel wire of the present invention may be applied. In this case, a dry wire drawing, a patenting heat treatment, and a plating treatment can be carried out by a method similar to a conventional method.

EXAMPLES

The present invention will now be described in more detail by way of Examples.

Examples 1 to 3 and Comparative Examples 1 to 6

Evaluation was carried out using steel wires having a wire diameter of 0.24 mm and tensile strengths listed on Tables 1 and 2. Steel wires subjected to the evaluation were manufactured according to conditions listed on Tables 1 and 2 below. For each steel wire, the shear strength exhibition index was calculated according to the following procedure. The obtained results are listed on the same Tables in combination. The occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction was measured by using D8 DISCOVER manufactured by Bruker Corporation.

<Measurement of Crystal Texture in [110] Orientation>

In measurement of EBSD, the magnification was set to 5000 times and 50 nm pitch, and seven visual fields were measured from the outside to the center of a longitudinal cross-section of a carbon steel wire. FIG. 2 is a longitudinal cross-sectional view of a carbon steel wire indicating measurement positions of the EBSD, and the seven areas surrounded by the dotted lines in the figure are seven visual fields to be measured. From the obtained results, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in a surface layer portion and a central portion and the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in a region from the outer periphery to 0.4r were calculated. FIG. 3 is a graph illustrating relationships between distances from the surfaces of carbon steel wires of Example 1 and Comparative Example 2 and the occupancy ratios of a crystal texture in the [110] orientation with respect to the longitudinal direction of the carbon steel wires.

<Shear Strength Exhibition Index>

A steel wire was held in a bent state at 165°, and a jig as illustrated in FIG. 4 was pressed against this steel wire, thereby obtaining the breaking strength of the steel wire, and the shear strength exhibition index of the steel wire was calculated from the ratio thereof to the simple tensile strength. FIG. 4 is a schematic explanatory view of a method of measuring the shear breaking strength of a steel wire. The R of a jig 4 illustrated in the figure was set to 0.2 mm. A steel wire with a tensile strength of 3,500 MPa was represented by an index with Comparative Example 1 taken as 100, a steel wire with a tensile strength of 3,750 MPa was represented by an index with Comparative Example 5 taken as 100, and a steel wire with a tensile strength of 3,200 MPa was represented by an index with Comparative Example 6 taken as 100. In Tables 1 and 2, they were expressed as the shear strength exhibition index.

TABLE 1 Comparative Comparative Comparative Comparative Example 1 Example 1 Example 2 Example 3 Example 4 Tensile strength (MPa) 3500 3500 3500 3500 3500 Tensile strength/number of 152 167 152 159 152 dies used Die drag*¹ 718 815 908 753 827 Occupancy ratio (%) of 84 84 88 84 87 crystal texture in [110] orientation of central portion Occupancy ratio (%) of 49 65 68 61 67 crystal texture in [110] orientation of surface 0.4r region*² Shear strength exhibition 108 100 100 101 100 index

TABLE 2 Comparative Comparative Example 2 Example 5 Example 3 Example 6 Tensile strength 3200 3200 3750 3750 (MPa) Tensile strength/ 145 168 152 165 number of dies used Die drag*¹ 672 827 705 837 Occupancy ratio (%) 80 82 92 88 of crystal texture in [110] orientation of central portion Occupancy ratio (%) 45 67 55 72 of crystal texture in [110] orientation of surface 0.4r region*² Shear strength 105 100 110 100 exhibition index

From the above Tables 1 and 2 and FIG. 3, it is known that the carbon steel wires of the present invention exhibit more excellent shear strength exhibition indexes than those of conventional ones, and that the carbon steel wires of the present invention are more excellent in shear resistance than conventional ones.

DESCRIPTION OF SYMBOLS

-   1 carbon steel wire -   2 surface layer portion -   3 central portion -   4 jig 

1. A carbon steel wire having a wire diameter of from 0.1 to 0.6 mm, characterized in that, when the radius of a circular cross-section orthogonal to the longitudinal direction is r and a region from the outer periphery of the circular cross-section toward the center to 0.4r is a surface layer portion, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the surface layer portion is 60% or less.
 2. The carbon steel wire according to claim 1, wherein, when a region inside the surface layer portion in the circular cross-section is a central portion, the occupancy ratio of a crystal texture in the [110] orientation with respect to the longitudinal direction in the central portion is greater than 60%.
 3. The carbon steel wire according to claim 1 which is for reinforcing rubber articles.
 4. The carbon steel wire according to claim 2 which is for reinforcing rubber articles.
 5. A method of manufacturing a carbon steel wire comprising a step of subjecting a carbon steel wire rod to wet wire drawing processing by using a plurality of dies, characterized in that when the tensile strength of an eventually obtained carbon steel wire is T (MPa) and the number of dies used in the wet processing is n, a relationship represented by the following formula (1): T/n≦155 (MPa)  (1) is satisfied, and when the radius of the carbon steel wire rod before the wet wire drawing processing is d0 and the wire diameter of the carbon steel wire after the wet wire drawing is d1, the maximum value of a die drag at a die with a wire drawing distortion a represented by the following formula (2): ε=2×ln(d0/d1)  (2) of 1.5 or less is 750 MPa or less. 